The present invention refers to a new auxiliary power unit starting system for an aircraft; in particular, for those aircraft whose auxiliary power unit starting architecture uses a dedicated link between power source and the auxiliary power unit starter motor.
The auxiliary power unit (APU) is a gas turbine engine that supplies electrical and pneumatic power to the aircraft systems as an auxiliary or secondary source of power. The APU allows the aircraft to be autonomous of external electrical and pneumatic power sources on the ground and in-flight. The APU is managed by the Electronic Control Box (ECB), a full-authority engine controller that monitors and provides full self-protection. The APU turbomachinery is mounted in a dedicated fire compartment in the tail cone of the aircraft, the APU compartment, while the APU controller is traditionally installed in the pressurized fuselage.
The APU provides pneumatic power to permit main engine start and compressed air for cabin comfort through the Environmental Control System (ECS). Electrical and pneumatic power can be provided to the aircraft separately or in combination. Electrical power has priority over pneumatic power.
The APU turbomachinery is traditionally started by means of a starter motor, which is mounted in the gearbox. The starter motor can be a DC starter motor or an AC starter motor.
The DC starter motor 3 is normally supplied by two different DC electrical power sources in parallel: a battery and a transformer rectifier unit (TRU). These two DC electrical power sources can be either shared between the APU 2 and any other aircraft system (FIG. 1), or dedicated to the APU starting system (FIG. 2). FIG. 1 shows a shared DC power network 4 that comprises a battery 18 and a TRU 19. FIG. 2 shows two DC power networks 4a, 4b that each comprise a battery 18a, 18b and a TRU 19a, 19b respectively. The DC power network 4b is dedicated to the APU starting system, while the DC power network 4a is used for any other aircraft system.
In addition to the DC power network 4, the nose fuselage power center traditionally comprises an AC power network 6 supplied by an AC power source 24. The AC power network 6 feeds the transformer rectifier unit 18 via an AC bus 21.
The current traditional architecture of the APU DC starting system shown in FIG. 1 comprises a dedicated DC link 9 between the electrical distribution center placed at the nose section 25 and the APU DC starter motor 3 placed at the APU compartment 26. Specifically, the dedicated DC link 9 connects the DC power network 4 with the DC starter motor 3. For that, the DC link 9 is connected to a DC bus 22 fed by both the transformer rectifier unit 18 and the battery 19.
The starting torque and corresponding electrical power required to start the APU are unavoidably very high. For some aircraft models, it is not possible to use the batteries 18a installed in the front part of the aircraft 15, due to the excessive heat dissipation and corresponding voltage drop in the electrical feeders connecting the batteries to the DC starter motor 3. Thus, as shown in FIG. 2, the battery 18b exclusively used to supply the APU DC starter motor 3 must be installed at the rear part of the aircraft 15, as close as possible to the APU compartment 26. For that, the DC power network 4a of the front part of the aircraft 15 is duplicated at the rear part of the aircraft 15.
The alternative APU DC starting architecture of FIG. 2 comprises a dedicated AC link between the electrical distribution center of the nose section and the duplicated DC power network 4b. Specifically, the dedicated AC link 27a connects the AC power source 24 by means of an AC bus 21, with the transformer rectifier unit 18b of the duplicated DC power network 4b. Additionally, the DC bus bar 22b of the duplicated DC power network 4b is connected to the APU DC starter motor 3.
Let it be noted how both traditional APU starting systems, respectively shown in FIGS. 1 and 2, imply a dedicated link (9, 27a and 27b) between the electrical distribution center at the nose section and the APU starter motor through the entire length of the aircraft 15.
It would therefore be desirable to provide technical means that simplify conventional APU starting systems, at the same time that reduce the associated weight while maximizing commonality and reuse of components from the electrical architecture on existing aircraft.
Additionally, it would be desirable to reduce the cost associated to traditional APU starting systems.